1. Technical Field
The invention is concerned with the economical fabrication of a variety of products entailing sol-gel produced high silica glass. In broad terms, the inventive advance is low-cost. Resulting product may have properties equal to those of product as produced by more expensive methods. The same consideration may, however, dictate use of silica glass, as now economically feasible, in lieu of other material, e.g., of low melting mixed oxide glass, to, in turn, manifest improved properties.
An important thrust concerns optical quality--the inventive processes permit, not only improved strength, stability and other properties associated with silica, but may avoid adverse optical effects as due, e.g., to porosity or bubbles. An important field addressed concerns optical fiber. While fabrication of the entirety of the fiber is contemplated as well, an important use involves fiber drawing from a composite preform consisting of a core rod enclosed within an overcladding tube. The overcladding tube is produced by sol-gel--the core rod by deposition using MCVD or a soot process.
2. Terminology
The arts involved, commercial as well as scientific, make use of a variety of terms which are not consistently applied, or which have some non-general, art-acknowledged meaning. The following definitions are largely in terms of a primary thrust of the invention--that of optical fiber.
Fumed Silica
Thus, the likely starting material for use in the sol-gel processing of the invention, is prepared by flame hydrolysis of an appropriate silicon compound, e.g. silicon tetrachloride, generally using an oxyhydrogen flame to yield particulate silica (silica powder).
Colloidal Silica
This terminology has the usual art-assigned meaning of particles of such size/mass as not to settle in some permitted period of time as included in suspension--so permitting formation of the required sol. The inventive process requires only such particle distribution/separation as to maintain the sol for the time required for gelation--under specified conditions. As a consequence, requirements for the transitory sol state may be somewhat more relaxed than in other contexts.
Sol
A dispersion of colloidal particles in a liquid.
Gel
A sol which has been gelled so as to be essentially non-fluid. Gels consist of a network of bonded colloidal sol particles, initially with some still-present, interstitial, liquid. Processing contemplates removal of such liquid so as to result in a "dried gel".
Core Rod
A glass cylinder consisting of a core material surrounded by sufficient primary cladding material so as to, together, form the part of the preform yielding the primary, optically functional portion of the ultimate fiber. The core rod may be produced by vapor deposition methods now used in fiber fabrication--e.g. MCVD, VAD, OVD.
Overcladding
The core rod is inserted within an overcladding tube to yield a hybrid preform for fiber drawing. The overcladding is silica glass which may be doped to depress refractive index. The overcladding tube is of size to yield a fiber of desired properties upon drawing from the composite preform. Typically, the overcladding comprises at least 80-90% of the fiber preform volume. The invention provides for a sol-gel produced overcladding tube as direct replacement for now-available overcladding tubes prepared by other methods. Accordingly, such a tube, likely to be used in initial commercialization of the invention, is consolidated prior to insertion of the core rod. Alternatively, likely descriptive of later commercialization, the tube may be unconsolidated--may be still porous--upon insertion of the rod, in which event consolidation is attendant upon collapse during formation of the hybrid preform.
Description of the Prior Art
Silica glass, while generally more expensive than many other inorganic glasses, finds many uses based on its excellent properties. These include transparency, chemical inertness, low thermal expansion, thermal stability, and strength. Well-known uses for silica glass, as based on such properties, include optical fiber; optical elements--lenses and mirrors; beakers, muffles, crucibles and other containers profiting by chemical and thermal stability; and windows for use in high temperature environments, e.g., partitioning windows between regions of different temperatures, in which advantage is taken of tolerance not only for high temperature, but for significant temperature gradients as well.
A major thrust of the invention concerns silica glass as used in optical fiber. After some considerable worldwide effort directed to fabrication of fiber by use of low melting mixed oxides, the many advantages of silica glass were recognized as justifying the added material/processing cost.
Efforts at cost saving with regard to presently produced fiber--low insertion loss, minimum dispersion fiber--have taken account of the fact that a major portion of the fiber--particularly of the now-prevalent single mode fiber--is made up of material of little optical performance significance. Considering that the functioning part of the fiber--the core and inner cladding carrying 99+% of the optical energy--typically consists of but 5% of the mass, a significant part of this effort has concerned structures providing for overcladding of such inner portion. Advanced manufacture at this time often makes use of an inner portion constituting core and inner clad region as fabricated by Modified Chemical Vapor Deposition, or, alternatively, by soot deposition in Outside Vapor Deposition or Vapor Axial Deposition. This core rod is overclad by material of less demanding properties, and, consequently, may be produced by less costly processing. Overcladding may entail direct deposition on the core rod, or may result from collapsing an encircling tube. Such "overcladding" tubes, at this time, are commercially produced from soot or fused quartz.
Workers in the field are well aware of the economy to be realized in overcladding fabrication by use of an alternative procedure--by use of "sol-gel". This well-known procedure described, for example, in J. Zarzycki, "The Gel-Glass Process", pp. 203-31 in Glass: Current Issues, A. F. Wright and J. Dupois, eds., Martinus Nijoff, Boston, Mass. (1985), is seen as far less costly than procedures now in use. While the literature shows extensive worldwide effort, to date sol-gel has not found commercial use in fiber fabrication. The explanation generally entails cracking as occurring in the formation of expediently sized preform overcladding tubing, or, alternatively, the involved and expensive processing to which resort has been had for avoiding such cracking. A representative reference, T. Mori, et al, "Silica Glass Tubes By New Sol-Gel Method", J. Non-Crystalline Solids, 100, pp. 523-525 (1988), first alludes to the cracking problem, and then describes a crack-avoiding process entailing a starting mixture and forming process, both of which are involved and expensive. The example given in the paper yields a 300 gram body--in any event somewhat smaller than generally desired.
The cracking problem is emphasized in a recent paper by Katagiri and Maekawa, J. Non-Crystalline Solids, 134, pp. 183-90, (1991) which states, "One of the most important problems in the sol-gel preparation method for monolithic gels is avoidance of crack formation which occurs during drying". A 1992 paper published in the Journal of Material Science, vol. 27, pp. 520-526 (1992) is even more explicit: "Although the sol-gel method is very attractive, many problems still exist, as pointed out in Zarzycki. Of these problems, the most serious one is thought to be the occurrence of cracks during drying of monolithic gel". The reference then reviews remedies, e.g. hypercritical drying procedures and use of chemical additives such as N,N dimethylformamide, collectively referred to as Drying Control Chemical Additives. Both methods are regarded as expensive and, therefore, undesirable in routine glass production. The conclusion is that there is presently no satisfactory technique for the economic preparation of large glass bodies from gel.
To circumvent the cracking problem particularly in the preparation of large silica bodies, workers have attempted, not only supercritical drying and DCCA's, but even more sophisticated routes. (See, e.g. R. Dorn, et al., "Glass from Mechanically Shaped Preforms", Glastech, Ber., vol. 66, pp. 29-32 (1987) and P. Bachmann, et al, "Preparation of Quartz Tubes by Centrifugal Deposition of Silica Particles", pp. 449-53 in Proceedings of the 14th European Conference on Optical Communications, Brighton, UK, IEE, Lond, U.K. (1988).
Small amounts of uncracked glass have been made by use of sol-gel. Examples are thin films of total mass of a fraction of a gram and small bodies weighing a few to a few hundred grams. Aspheric lenses and waveguide components (U.S. Pat. No. 5,080,962) are examples of small silica glass articles making use of sol-gel.
Approaches resulting in crack-free small bodies have not proven to be adequate for the larger bodies often required to realize the cost-saving promise of sol-gel. For example, expedient fiber fabrication generally entails drawing of tens of kilometers to hundreds of kilometers from a single--preform without the time and expense entailed in preform replacement. The desire for crack-free sol-gel preform tubes of requisite size--weighing a kilogram or more--has not been satisfied. The most relevant prior art is represented by U.S. Pat. No. 4,775,401, "Method for Producing Optical Fibers". The patent teaches direct overcladding of a core-rod by a dried sol-gel derived tube. It describes use of quaternary ammonium hydroxides to stabilize the sol followed by the use of an ester to bring about controlled gelation. An example in that patent speaks of the successful preparation of a crack-free 300 gram silica cylinder.
Efforts in this direction have included a number of other approaches, e.g., those of U.S. Pat. Nos. 4,059,658 (R. D. Shoup, et al) and 3,827,893 (H. Meissner, et al). These processes are based on precipitation of silica particles from solution. Some such processes depend upon use of solutions containing potassium silicate source material. The latter, as applied to more demanding use in turn requires leaching to remove the crystallization-inducing alkali metal ion.
Bachman, et al, cited above, depends upon a suspension of silica particles as initially introduced (rather than by precipitation) and depends upon centrifugal particle deposition. Problems associated with gelation are avoided.
Dorn, et al sidestep problems associated with sol-gel altogether by resorting to mechanical compaction of dry powder.